Color measurement instruments fall into two general categories: wideband (or broadband) and narrowband. A wideband measurement instrument reports up to 3 color signals obtained by optically processing the input light through wideband filters. Photometers are the simplest example, providing a measurement only of the luminance of a stimulus. Their primary use is in determining the nonlinear calibration function of displays. Densitometers are an example of wideband instruments that measure optical density of light filtered through red, green and blue filters. Colorimeters are another example of wideband instruments that directly report tristimulus (XYZ) values, and their derivatives such as CIELAB. A colorimeter, sometimes also called an imaging photometer, is an imaging device which behaves like a camera. The imaging colorimeter can be a time-sequential type or Bayer-filter type. Under the narrowband category fall instruments that report spectral data of dimensionality significantly larger than three.
Spectrophotometers and spectroradiometers are examples of narrowband instruments. These instruments typically record spectral reflectance and radiance respectively within the visible spectrum in increments ranging from 1 to 10 nm, resulting in 30-200 channels. They also have the ability to internally calculate and report tristimulus coordinates from the narrowband 15 spectral data. Spectroradiometers can measure both emissive and reflective stimuli, while spectrophotometers can measure only reflective stimuli. A spectrometer or spectrograph is a narrowband device which can quantify and measure the spectrum.
The main advantage of wideband instruments such as densitometers and colorimeters is that they are inexpensive and can read out data at very fast rates. However, the resulting measurement is only an approximation of the true tristimulus signal, and the quality of this approximation varies widely depending on the nature of the stimulus being measured. Accurate colorimetric measurement of arbitrary stimuli under arbitrary illumination and viewing conditions requires spectral measurements afforded by the more expensive narrowband instruments. Compared with measuring instruments without spatial resolutions, such as spectrometers, this technology offers the following advantages: (a) Substantial time-savings with simultaneous capture of a large number of measurements in a single image and (b) Image-processing functions integrated in the software permit automated methods of analysis, e.g. calculation of homogeneity or contrast.
However, the absolute measuring precision of imaging photometers and colorimeters is not as high as spectrometers. This is because of the operational principle using a CCD Sensor in combination with optical filters, which can only be adapted to the sensitivity of the human eye with limited precision. Therefore, the imaging colorimeters are the instruments of choice for measurement of luminance and color distribution of panel graphics and control elements in the display test industry, including but not limited to homogeneity, contrast, mura and modulation transfer function (MTF).
Therefore, what is desired is an alternative to wideband colorimeters that can include more accurate outputs.